There is a conventional circulating fluidized bed furnace wherein circulation medium is heated by combustion in a riser (a fluidized combustion furnace) and combustion gas blown up is laterally introduced through a lateral duct into a cyclone separator for capture of the circulation medium, the captured circulation medium being guided to and stored in a storage. The stored circulation medium is fluidized for circulation into the riser (see Patent Literature 1).
In Patent Literature 1, the fluidization of the circulation medium stored in the storage is controlled to control a flow rate of the circulation medium to be circulated to the riser. In Patent Literature 1, the amount of the circulation medium taken out from the riser and supplied to the storage is controlled, for example, by regulating a superficial velocity in the riser through control of the supply of air to the riser.
Meanwhile, nowadays, there is a circulating fluidized bed gasification furnace wherein circulation medium is heated in a riser and the heated circulation medium is captured in a cyclone separator. The captured circulation medium is guided to the fluidized bed gasification furnace where heat possessed by the guided circulation medium is utilized for a gasification reaction (endothermic reaction) of a raw material. The circulation medium lowered in temperature due to the gasification and ungasified unreacted char are circulated to the riser where the char is burned to heat the circulation medium (see Patent Literature 2).
FIGS. 1 and 2 are schematic views showing an example of the above-mentioned circulating fluidized bed gasification furnace. Reference numeral 1 denotes a riser to which air 12 is supplied for combustion; 2, a cyclone separator to which combustion gas 4 taken out through a top of the riser 1 via a lateral duct 3 is guided, circulation medium admixed in the combustion gas 4 being captured while exhaust gas 6 is discharged; 7, a fluidized bed gasification furnace to which the circulation medium 5 captured by the separator 2 and raw material 8 such as coal are guided and to which a gasifying agent 9 such as steam or air is introduced from below, heat of the circulation medium 5 being utilized for gasification of the raw material 8 to produce gasification gas 10; 11, a circulation passage for returning the circulation medium and ungasified unreacted char in the fluidized bed gasification furnace 7 to the riser 1 where the char supplied through the passage 11 is burned to heat the circulation medium. In FIG. 1, reference numeral 13 designates auxiliary fuel.
In the circulating fluidized bed gasification furnace shown in FIGS. 1 and 2, plenty of heat must be supplied by the riser 1 for the gasification reaction in the fluidized bed gasification furnace 7. To this end, a supply of the circulation medium 5 from the riser 1 to the fluidized bed gasification furnace 7, i.e., an amount of the circulation medium 5 taken out from the riser 1 is required to be increased to balance heat budget in the riser 1 and gasification furnace 7 to thereby enable gasification.
Thus, it is conceivable also in the apparatus shown in FIGS. 1 and 2 that, just like Patent Literature 1, the supply of air 12 to the riser 1 is increased to enhance superficial velocity in the riser 1 to thereby increase the supply of the circulation medium 5 from the riser 1 to the fluidized bed gasification furnace 7.
However, a given reaction time is required for combustion of char in the riser 1; mere enhancement of the superficial velocity in the riser 1 by increase of the air amount would bring about deteriorated combustion quality, possibly resulting in the insufficiently heated circulation medium. Thus, even if the supply of air 12 to the riser 1 is controlled to obtain an utmost superficial velocity which ensures sufficient combustion quality, increase in supply of the circulation medium from the riser 1 to the fluidized bed gasification furnace 7 is limited. Specifically, as shown in FIGS. 1 and 2, in order to increase a flow velocity of the combustion gas 4 tangentially supplied to the cyclone separator 2 for separation of the circulation medium 5, the lateral duct 3 interconnecting the riser 1 and the cyclone separator 2 must have a reduced cross-sectional area and thus has a cross-sectional area smaller than that of the riser 1.
As a result, the circulation medium blown up to an upper end of the riser 1 has difficulty in orientation to the lateral duct 3, and may drop in the riser 1 by itself or through collision against a top wall of the riser 1, failing to increase a circulated amount of the circulation medium from the riser 1 to the fluidized bed gasification furnace 7. Such failure of the increased circulation medium 5 from the riser 1 to the fluidized bed gasification furnace 7 may result in lack of heat necessary for gasification of the raw material 8 in the fluidized bed gasification furnace 7. In order to overcome this, the auxiliary fuel 13 must be supplied to the riser 1 to enhance a heating temperature of the circulation medium; alternatively, the riser 1 must be increased in size to increase the circulated amount of the circulation medium 5 through increase of a total amount of the circulation medium. As a result, disadvantageously, running or equipment cost will increase.
In order to overcome such problem, there has been proposed an apparatus wherein at least a reduced-diameter intermediate cylinder is arranged on a top of a riser to increase a flow velocity of combustion gas in the riser to thereby increase an amount of the circulation medium discharged to a cyclone separator (see Patent Literature 3).